1. Field of the Invention
The present invention relates to a physical parameter sensor for detecting such physical quantities as acceleration, angular velocity, and pressure, and relates more particularly to a physical parameter sensor for processing a signal in reference to a reference frequency.
2. Description of the Prior Art
FIG. 11 is a block diagram of the prior art physical parameter sensor for detecting angular velocity. As shown in FIG. 11, this physical parameter sensor 200 comprises a sensor circuit 201, a signal detection circuit 202, a signal processing circuit 203, and an oscillation circuit 204.
The oscillation circuit 204 generates a sine wave signal having a particular reference frequency, and outputs this reference frequency signal to the sensor circuit 201 and to the signal detection circuit 202. The sensor circuit 201 operates based on the frequency of this reference frequency signal, inputted from the oscillation circuit 204, to convert a detected physical quantity into an electric signal. A signal indicative of the detected physical quantity is then outputted from the sensor circuit 201 to the signal detection circuit 202.
The signal detection circuit 202 amplifies this signal indicative of the detected physical quantity and then rectifies the amplified signal to output a direct current (DC) signal. The resulting DC physical quantity signal is supplied to the signal processing circuit 203. It should be noted that the signal detection circuit 202 utilizes the reference frequency signal, which has been supplied from the oscillation circuit 204, when rectifying the physical quantity signal to the DC signal.
The signal processing circuit 203 processes the DC signal to compensate for a temperature-dependent change of the DC signal from the signal detection circuit 202 and then amplifies the compensated signal. The amplified signal is outputted from the signal processing circuit 203 as an output signal of the physical parameter sensor 200 through an output terminal OUT. The output signal from the physical parameter sensor 200 is subsequently supplied to a control circuit 8 which typically may be a microcontroller or any other semiconductor device, used to execute a particular process based on the signal inputted from the physical parameter sensor 200.
The output signal from the signal processing circuit 203, i.e., the output signal from the physical parameter sensor 200, is called an "offset output" when the detected physical parameter is 0. This offset output is set to Vcc/2 where Vcc represents the voltage of a direct current power source. Accordingly, when the reference frequency signal is not supplied from the oscillation circuit 204 to the sensor circuit 201 for some reason, the level of the signal inputted from the sensor circuit 201 to the signal detection circuit 202 is 0 and, therefore, the output signal from the signal processing circuit 203 is Vcc/2.
Consequently a voltage within the normal output voltage range will be outputted from the output terminal OUT of the physical parameter sensor 200 even when the sensor circuit 201 is unable to detect the desired physical parameter because of some problem in the oscillation circuit 204 or breakage of an internal wiring of the physical parameter sensor 200. In other words, the prior art physical parameter sensor 200 has no self-testing capability.